OLED-based displays are currently being considered for use in many applications which presently employ liquid crystal displays (LCDs). OLED-based displays can provide brighter and clearer images than liquid crystal displays and also need less power. However, the organic molecules used in OLEDs are strongly reactive with oxygen and moisture, and thus susceptible to damage from exposure to air. Such exposure may lead to a reduction in the useful life of the light emitting device. Therefore, hermetic sealing is one of basic requirements for long term performance of OLEDs.
Efforts have been made to hermetically seal OLED-based displays with organic materials, such as epoxy resins. An alternate technology with substantially better performance has been developed by Corning Incorporated, the assignee of this application. In accordance with this approach, a frit-containing paste is made by mixing glass particles, filler particles, e.g., crystalline particles, and a vehicle, e.g., a vehicle comprising one or more solvents and one or more binders and/or dispersing aids. The paste is dispensed on a first substrate (e.g., a first glass sheet) and is sintered using, for example, a high temperature furnace to produce a sintered frit pattern.
The resulting assembly, known as a fritted cover glass or simply a cover, is combined with a second substrate (e.g., a second glass sheet) carrying one or more OLED devices. The cover and the second substrate are sealed together by exposing the sintered frit pattern to laser energy. In particular, a laser beam is scanned (traversed) over the sintered frit pattern to locally raise the temperature of the sintered frit above its softening point. In this way, the sintered frit adheres to the second substrate and forms a strong seal between the cover and the second substrate. Since the sintered frit is a glass and ceramic material, as opposed to an organic material, the penetration of oxygen and moisture through the frit seal is much slower than through the epoxy seals previously used to encapsulate OLED devices.
The sintered frit sealing technique, however, does have the disadvantage that it uses a high power laser to melt the sintered frit. The resulting thermal cycle can cause thermal damage to OLED devices, a problem which does not usually arise with epoxy sealing employing ultraviolet (UV) curing. Also, in the laser frit sealing technique, the sintered frit needs to be bonded to various device materials such as metal leads, indium tin oxide (ITO), protective materials, and the like. In addition, each material on the device side of the sintered frit has a different set of thermal properties (e.g., coefficient of thermal expansion (CTE), heat capacity and thermal conductivity). These different materials and different sets of thermal properties can cause a significant variation in required buffer space to achieve a strong bond of the sintered frit without creating thermal damage to the OLED.
To minimize these problems, an OLED-based display typically includes a substantial border, e.g., a border having a width of 600-1500 microns, between the OLED device(s) and the inner edge of the sintered frit (referred to herein as the “inner unused area”). For small displays, such as those used in cell phones, PDAs and other mobile electronic devices, this inner unused area represents a substantial fraction of the total area available for image generation.
A further limitation on useable space arises from the manner in which OLED-containing glass packages are processed after laser sealing has been completed. Specifically, when the sealing step is finished, the device is typically scored and broken to a desired dimension (the resulting package is referred to herein as a “sized package”). In practice, it has been found that if the score line is too close to the edges of the sintered frit, the frit will be exposed to a high level of stress during the scoring and breaking process which will weaken the bond strength significantly and/or cause delamination. It is also difficult to achieve glass edges free of defects without damaging the sintered frit after scoring if the score line is too close to the sintered frit edge.
For these reasons, as well as to accommodate the tolerances of commercial scoring machines, a minimum distance in the range of 300 to 600 microns from the score line to the sintered frit edge has been maintained on three sides of sized OLED-based displays, the specific distance depending on the particular equipment being used and display being produced. (The fourth side is used to make electrical connections to the OLEDs and is normally left larger than the other three sides). This three-sided outer border is referred to herein as the “outer unused area” and, along with the inner unused area discussed above, represents a substantial fraction of the sized packages of small displays, e.g., displays having a viewing area in the range of 1.5 to 20 square centimeters.
Small displays also present challenges with regard to strength since, as is well known, such displays are often dropped, sat on, bumped, and otherwise abused in the field. Compared to an epoxy resin, sintered frits, being glass/ceramic materials, are less flexible. It would thus be desirable to increase the basic strength of the sintered frit seal to minimize its chances of failure in the field. In particular, it would be desirable to increase the width of the sintered frit in order to provide a larger bonded area and thus greater overall mechanical strength. However, given the limited areas available as a result of the inner and outer unused areas, dedicating more space to the sintered frit wall has been resisted by OLED display manufacturers.
In view of the foregoing, there exists a need in the art for electronic packages, such as OLED-based display packages, which have reduced unused areas. The present invention addresses this need.